1. Field of the Invention
The present invention relates to a method for measuring an amount of toner on a recording medium, a method for image formation, a toner amount measuring apparatus, and an image forming apparatus, and more particularly, to a method for measuring the amount of toner fixed to a recording medium by an image forming apparatus.
2. Description of the Related Art
Conventional copying machines include MFPs (Multi Function Printers) which provide the functions of a printer or a copying machine and which can be connected to a network. In many cases, between such an MFP and an apparatus connected to the MFP via a network, color matching for an image to be printed is carried out or the colors of an image displayed on a display such as a CRT are matched with the colors of an image to be printed. Conventionally, a variety of color management techniques have been disclosed for color matching as described above.
For example, in color management using ICC (International Color Consortium) profiles, an ICC profile unique to an apparatus such as a printer or a copying machine is created so as to enable color matching (calibration or characterization) on the basis of the created ICC profile. Specifically, print data created by a personal computer (hereinafter referred to as a PC) is subjected to a color conversion using an ICC profile. The color-converted data is output to the apparatus corresponding to the ICC profile to match colors of an image to be printed with colors of an image displayed on a display or the like of the PC.
Software that creates a profile for calibration and color measuring instruments has been provided. Thus, environments are being prepared in which users can match colors output by an image forming apparatus such as a printer with target colors.
Some conventional calibrations involve changing the contents of a gamma LUT for gradation without any color conversion using a multi dimensional LUT (gradation correction table) for ICC profiles to obtain desired gradation characteristic.
As described above, color management can suppress a difference in output color between a plurality of apparatuses of the same type or of different types. The scope of applications of the color management is not limited to those described above. For example, the color management is also applicable to the case where colors to be printed by an offset printing machine are matched with colors printed by a printer to allow the printer to be used for color calibration for printing carried out by the offset printing machine. Preparing respective ICC profiles for the offset printing machine and for the printer can realize such color management as shown in FIG. 18 on an application in the PC.
In FIG. 18, the contents of a printing machine ICC profile 510 and of a printer ICC profile 520 are calibrated in association with a color space that does not depend on the printing machine or printer, for example, a CIEL*a*b* color space, on the basis of color measurements of patch images using a color measuring instrument. This enables colors printed by the printing machine to be matched with those printed by the printer. A color management module (CMM) 530 can use these profiles to carry out color conversions to create print data.
Moreover, in the printing industries, it is pursued to heighten the added values of output devices such as printers. For example, the added values have been heightened by using special colors for photographic printing. Specifically, high quality printing with a reduced granular impression and a widened color reproduction range has been achieved by using gray ink as well as orange, violet, and the like as special colors in addition to CMYK basic colors. Ink jet printers using light and special colors and capable of controlling gloss have been provided in order to accomplish high printing quality.
Color management techniques for color measuring instruments and profile creating software need to be improved in order to heighten the added values of output devices. Software capable of creating a profile for up to 10 colors has been provided.
On the other hand, color adjustment by a printer engine involves outputting a patch image with a monochromatic gradation, allowing a reader section to read the patch image to calculate its density, and creating a one-dimensional LUT so as to obtain a desired target (linear density, linear lightness, or the like) as described in, for example, Japanese Laid-Open Patent Publication (Kokai) No. H11-075067. In this color adjustment, white light emitted by the reader section is incident on the output patch image, and the resulting irregularly reflected light is incident on a CCD sensor to allow the image density of the patch image to be detected.
Further, Japanese Laid-Open Patent Publication (Kokai) No. 2002-072574 discloses a technique for forming a patch image on a transfer unit, detecting the application amount (density) of toner (recording material) on the patch image formed using a regular reflection sensor, and feeding back the detected application amount to an LUT or ATR for color adjustments. This can maintain the stability of colors without troubling the user.
Moreover, Japanese Laid-Open Patent Publication (Kokai) No. 2003-215981 discloses a technique for calculating outputs from an irregular reflection sensor and a regular reflection sensor to automatically control the amount of toner used to an optimum value.
For the amount of transparent toner which is difficult to detect using irregularly reflected light, Japanese Laid-Open Patent Publication (Kokai) No. 2005-275250 discloses a technique for using the amount of regularly reflected light from an output image to detect macroscopic glossiness to control the toner amount.
Further, for special colors, Japanese Laid-Open Patent Publication (Kokai) No. 2005-280358 discloses a technique for calculating the recording material amount (the amount of ink applied) on the basis of spectral reflectance data.
However, in order to detect the amounts of toner for all of CMYK, special colors, and transparence, the conventional methods for color adjustment has required not only the optical sensor arrangement for outputting regularly reflected light and irregularly reflected light but also the spectral reflectance sensor for irregularly reflected light, which detects the amount of toner of a special color. This increases apparatus costs and requires a large space in which the sensors are installed, unavoidably increasing the size of the apparatus.
With the method for detecting the density utilizing the light absorption of a toner as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-075067, the accuracy of high density detecting has been insufficient. Two factors contributing to this insufficiency are a method for detecting a toner and a method for image formation.
The reason why the detecting method is a factor will be described in connection with a method for calculating density. The density is measured using the equation shown below, where I1 denotes the intensity of reflected light and I0 denotes the intensity of incident light.Density=−log(I1/I0)
At a density of 1.0, 10% of incident light is reflected. However, at a high density of 2.0, only 1% of incident light is reflected. FIG. 19 is a diagram showing the relationship between the density of a toner and detected luminance. The axis of abscissa indicates the density, and the axis of ordinate indicates the detected luminance (100×(I1/I0)%). In FIG. 19, reading resolution is set at 8 bits. FIG. 20 is a diagram showing the relationship between the density and the detected luminance with a high density part focused on. As shown in FIG. 20, in the high density part, a variation of 0.5% in detected luminance, that is, a variation of 0.5% in detecting accuracy, varies the density by as much as 0.1. Thus, the conventional method for detecting the density of a toner has not been able to accurately detect the density of a toner particularly at a high density.
The reason why the method for image formation is a factor lies in toner and a fixation system. With an electrophotographic scheme, toners are stacked and melted to develop colors. However, the amount of toner exceeding a specified value prevents an increase in density value (see FIG. 21). The amount of toner exceeding the specified value results in uneven gloss, an unfixed image, or a toner release phenomenon called offset, preventing an increase in density value.
Thus, image forming apparatuses have adjusted colors by measuring the density of an image and adjusting the amount of toner used so as to set the density at a desired value. However, this has resulted in a decrease in the accuracy with which the density of the high density part is detected and a mismatch with the amount of toner used.
Similarly, with the methods for detecting the toner amount disclosed in other publications than the above, the accuracy with which the density of the high density part is detected has been low.
Moreover, with the method for detecting the toner amount using glossiness as described in Japanese Laid-Open Patent Publication (Kokai) No. 2005-275250, it is difficult to accurately control the toner amount without optimizing the type and fixing conditions of paper, the type of toner, the range of potential settings, and the like. Without optimization, the amount of toner used varies. A decrease in the amount of toner used reduces the density and thus the dynamic range, degrading image quality. On the other hand, an increase in the amount of toner used may cause an image defect such as an unfixed image in which the toner is released.
Under these circumstances, it has been desirable to establish a method for accurately measuring the toner amount in order to maintain the specified amount of toner used and to carry out control using the method.